Conventionally, as a method of inspecting a data packet stream on a computer network in order to search for a pattern divided by a border of a data packet, there are proposed methods involving determining whether or not two or more data packets are consecutive in a data packet stream, joining payloads from consecutive data packets, analyzing the joined payloads from the consecutive data packets, and searching for a plurality of patterns constituted by combinations of characters as well as a method involving returning a data packet to an output data stream based on predetermined determination criteria when the data packet is present in a system for a predetermined time or longer (refer to Japanese Translation of PCT Application No. 2009-510815).
In addition, conventionally, there are proposed, in a zone center station connected to a group center station accommodating a plurality of subscriber terminals connected to the Internet, a storage device which temporarily stores packet data transferred to and from the Internet for each user ID of the subscriber terminals and a gateway provided with functions for assembling a plurality of pieces of packet data stored in the storage device into file data by associating each piece of packet data with an IP address in the Internet and with a local IP address in a network, determining whether or not a computer virus exists in the assembled data, and transferring data in which a computer virus is determined not to exist to the subscriber terminals (refer to Japanese Patent Application Publication No. 2001-256045).